Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in an organic solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative primary lithium cells with lithium metal anodes but having different cathodes, are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically AA size or AAA size cells, but may be in other size cylindrical cells. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.4 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons change the oxidation state of +2 for Fe+2 in FeS2 to 0 in elemental iron (Fe0) and the remaining 2 electrons change the oxidation state of sulfur from −1 in FeS2 to −2 in Li2S. In order to carry out the electrochemical reaction the lithium ions, Li+, produced at the anode must transport through the separator and electrolyte medium and to the cathode.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly at higher current drain over 200 milliAmp, the voltage is flatter for longer periods for the Li/FeS2 cell than the Zn/MnO2 alkaline cell as may be evident in a voltage vs. time profile. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is more clearly and more directly shown in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is a primary (nonrechargeable) cell can be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell.
One disadvantage of the Li/FeS2 cell is that a passivation layer can gradually buildup on the lithium anode surface principally during cell storage or intermittent rest. The chemical nature of the passivation layer and its rate of buildup on the anode surface can render the passivation layer deleterious and thereby adversely affect cell performance. However, it has been determined that the rate of buildup of the anode passivation can be retarded and the passivation layer stabilized by employing certain electrolyte systems for the Li/FeS2 cell. This includes considering specific solvent mixtures in combination with specific lithium salts as well as including additives to the electrolyte. It is thus possible to produce a passivation layer on the lithium anode for the Li/FeS2 cell which does not significantly compromise cell performance.
The cathode material may be initially prepared in the form of a slurry mixture, which can be readily coated onto a substrate, typically a metal substrate, by conventional coating methods. The electrolyte added to the cell is a suitable electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is non reactive, with the undischarged or partially discharged electrode materials (anode and cathode components) and also non reactive with the discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged or partially discharged) could gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a cell failure. Thus, the electrolyte used in a Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable in contact with discharged, partially discharged and undischarged electrode materials. The electrolyte solvent should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in Li2S product in the cathode.
Primary lithium cells are in use as a power source for digital flash cameras, which require operation at higher pulsed power demands than is supplied by individual alkaline cells. Primary lithium cells are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium (or lithium alloy, essentially of lithium), a cathode formed of a coating of cathode active material comprising FeS2 on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. A microporous polypropylene separator for a lithium cell is disclosed, for example, in U.S. Pat. No. 4,794,057. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421.
A portion of the spiral wound anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
The anode in a Li/FeS2 cell can be formed by laminating a layer of lithium metal or lithium alloy on a metallic substrate such as copper. However, the anode may be formed of a sheet of lithium or lithium alloy without any substrate.
The electrolyte used in primary Li/FeS2 cells is formed of a “lithium salt” dissolved in an “organic solvent”. The electrolyte must promote ionization of the lithium salt and provide for good ionic mobility of the lithium ions so that the lithium ions may pass at good transport rate from anode to cathode through the separator. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. Nos. 5,290,414 and 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li (CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; lithium methide, Li(CF3SO2)3C; lithium perchlorate LiClO4; lithium bis(oxalato)borate, LiBOB and various mixtures. In the art of Li/FeS2 electrochemistry lithium salts are not always interchangeable as specific salts work best with specific electrolyte solvent mixtures.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane (DX) in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimethoxyethane (DME). As given in the example the dioxolane and 1,2-dimethoxyethane (DME) are present in the electrolyte in substantial amount, i.e., 50 vol % 1,3-dioxolane (DX) and 40 vol % dimethoxyethane (DME) or 25 vol % 1,3-dioxolane (DX) and 75 vol. % dimethoxyethane (DME) (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N is also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethylisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19) with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,218,054 (Webber) is disclosed an electrolyte solvent system wherein dioxolane-based solvent and dimethoxyethane-based solvent are present in a weight ratio of about 1:3 (1 part by weight dioxolane to 3 parts by weight dimethoxyethane).
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed a specific preferred electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX) (63.05 wt %), 1,2-dimethoxyethane (DME) (27.63 wt %), and small amount of 3,5 dimethylisoxazole (DMI) (0.18 Wt %). (col. 6, lines 44-48) The electrolyte is typically added to the cell after the dry anode/cathode spiral with separator therebetween is inserted into the cell casing.
In US 2007/0202409 A1 (Yamakawa) it is stated with reference to the electrolyte solvent for the Li/FeS2 cell at para. 33: “Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, acetonitrile, dimethyl carbonate, and dipropyl carbonate, and any one of them or two or more of them can be used independently, or in a form of a mixed solvent.” Such statement is misleading, since the art teaches only specific combinations of electrolyte solvents will be workable for the Li/FeS2 cell depending on the particular lithium salt to be dissolved in the solvent. (See, e.g. above U.S. Pat. Nos. 5,290,414 and 6,849,360) The reference Yamakawa does not teach which combination of solvents from the above list are to be used with any given lithium salt.
Conventional FeS2 powders, for example Pyrox Red 325 powder from Chemetall GmbH, are conventionally available with pH raising additives therein to offset or retard any buildup in acidity of the powder. Such additives are believed to contain calcium carbonate (CaCO3) or calcium carbonate linked to other compounds. Such calcium carbonate is added to the FeS2 powder to retard the formation of acidic impurities within or on the surface of the powder as it is stored in ambient air and exposed to oxygen and moisture. This is regardless of whether the FeS2 is intended for use in cathode mixtures or other applications, for example, as an additive in manufacture of car brakes.
It is desired to find an electrolyte for the Li/FeS2 cell which improves ionic mobility of the lithium ions passing from anode to cathode and thus enhances cell performance and rate capability.
It is desired to find additives for the electrolyte which reduces the chance of polymerization of the electrolyte solvents and also retards the rate of buildup of deleterious passivation layer on the lithium anode.
It is desired to produce a primary (nonrechargeable) Li/FeS2 cell having good rate capability that the cell may be used in place of rechargeable batteries to power digital cameras.